While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pair and the resultant electroluminescence.
These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent multilayer organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm ) between the anode and the cathode. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. Because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays, these devices are now more attractive for many display applications. Tang et al., has described this multilayer OLED device in U.S. Pat. Nos. 4,769,292; 4,885,211 and in J. Applied Physics, Vol. 65, Pages 3610 -3616, 1989 which describe an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer wherein the light-emitting layer commonly consists of a host material doped with a guest material—dopant, which results in an efficiency improvement and allows for color tuning.
EL devices in recent years have expanded to include not only single color emitting devices, such as red, green and blue, but also white-devices, devices that emit white light. Efficient white light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
Since the early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569, 5,409,783, 5,554,450, 5,593,788, 5,683,823, 5,908,581, 5,928,802, 6,020,078, and 6,208,077, amongst others.
Notwithstanding all of these developments, there are continuing needs for organic EL device components, such as electron transporting materials and or electron injecting materials, that will provide even lower device drive voltages and hence lower power consumption, while maintaining high luminance efficiencies and long lifetimes combined with high color purity.
A useful class of electron transporting materials is that derived from metal chelated oxinoid compounds including chelates of oxine itself, also commonly referred to as 8-quinolinol or 8-hydroxyquinoline. Tris(8-quinolinolato)aluminum (III), also known as Alq or Alq3, and other metal and non-metal oxine chelates are well known in the art as electron transporting materials.
Tang et al., in U.S. Pat. No. 4,769,292 and VanSlyke et al., in U.S. Pat. No. 4,539,507 lower the drive voltage of the EL devices by teaching the use of Alq as an electron transport material in the luminescent layer or luminescent zone.
Baldo et al., in U.S. Pat. No. 6,097,147 and Hung et al., in U.S. Pat. No. 6,172,459 teach the use of an organic electron transporting layer adjacent to the cathode so that when electrons are injected from the cathode into the electron transporting layer, the electrons traverse both the electron transporting layer and the light emitting layer.
Tamano et al., in U.S. Pat. No. 6,150,042 teaches use of hole-injecting materials in an organic EL device. Examples of electron transporting materials useful in the device are given and included therein are mixtures of electron transporting materials. There is no indication of how to select the electron transporting materials in terms of Lowest Unoccupied Molecular Orbital levels (LUMOs) and no reference to low drive voltage with the devices.
Organometallic complexes, such as lithium quinolate have been used in EL devices, for example see WO 0032717 and US 2005/0106412. In particular, mixtures of lithium quinolate-and Alq have been described as useful, for example in U.S. Pat. No. 6,396,209 and US 2004/0207318. However, lithium quinolate, when used in an OLED device as the only electron-transporting material, results in a device with high drive voltage.
Seo et al., in US2002/0086180A1 teaches the use of a 1:1 mixture of Bphen, (also known as 4,7-diphenyl-1,10-phenanthroline or bathophenanthroline) as an electron transporting material, and Alq as an electron injection material, to form an electron transporting mixed layer. However, the Bphen/Alq mix of Seo et al., shows inferior stability and falls outside the scope of the current invention.
However, these devices do not have the desired EL characteristics in terms of luminance and stability of the components in combination with low drive voltages.
The problem to be solved therefore, is to provide an OLED device having a light-emitting layer (LEL) that exhibits good luminance efficiency and stability while at the same time requiring low drive voltages for reduced power consumption.